The quick answer is by dividing your opponent.
With the US elections a year or more away, the fight is on between the larger democratic base and the smaller (by numbers) republican base. I won’t delve into US partisan population data but instead talk about how the republicans effectively use their smaller base to win elections on a national scale. Part of that has to do with getting more people out to vote but a significant part of the campaign is dedicated to getting democrats to abstain.
And it is this second part that is soo interesting because it is a time tested strategy of divide and conquer. When colonial powers went to their soon-to-colonies they needed a strategy that would allow them to rule a large population with a small force. The strategy they used was “divide and conquer” essentially playing up all the small differences between local populations so that they would never band together and form a formidable collective opposition to the colonizers.
This same tactic was used by the Trump campaign in 2016 – by playing up the internal infighting between Hillary and Bernie they managed to fragment the democratic base and get likely democratic candidates to abstain or not vote for the democratic candidate.
Trump is already deploying the same tactics for 2020 by playing the democratic candidates against each other while ensuring that the republican base is united in their support for Trump.